1. Field of the Invention
The present invention relates to an electroconductive resin composition as a resin composition for use in electric wiring.
2. Description of the Related Art
Examples of an electroconductive resin composition which contains a low melting metal melting at an injection molding temperature are disclosed in Japanese Patent Publication No. Hei 7-49491 and Japanese Patent Application laid-open No. Hei 5-325637. These electroconductive resin compositions are respectively composed of a metal fiber, low melting metal and synthetic resin, and generally formed by injection molding or like method into predetermined configurations. The metal fiber and low melting metal as components are embedded in the synthetic resin in close contact with each other. Thus, resultant molded bodies exhibit electric conductivity.
The conventional electroconductive resin compositions, however, have problems of low fluidity and bad moldability, because the metal fiber has been used as a main component adapted to give electric conductivity. To improve the moldability, the metal fiber content may be reduced while increasing the low melting metal content. But, with this method, there arises another problem that phase separation of the low melting metal occurs upon molding to disenable the formation of the molded bodies which exhibit homogeneous electric conductivity. In addition, there also arises the problem that the metal fiber is too expensive to be used industrially.
It is an object of the present invention to provide an electroconductive resin composition which is excellent in both moldability and electric conductivity, and is produced at low costs.
The present inventors have noted that a metal fiber used conventionally as an electroconductive material degrades the moldability, and contemplated the use of a metal powder in place of the metal fiber. And they have found that the combination of a zinc metal powder and solder alloy containing tin as a main component exhibits excellent moldability and phase-separation resistance, and have completed the present invention. Namely, the present invention is characterized in that the electroconductive resin composition contains a zinc-based metal powder, low melting metal which melts upon molding, and synthetic resin material.
It can be considered that the zinc-based metal powder which has a large surface area captures the molten low melting metal to obstruct the phase separation of the low melting metal from both resin and zinc-based metal powder. Furthermore, the zinc-based metal powder has a round configuration so as not to damage the fluidity of the resin greatly. Thus, the electroconductive resin composition of the present invention exhibits excellent moldability.
As described above, the electroconductive resin composition of the present invention contains the zinc-based metal powder, low melting metal and synthetic resin material.
The zinc-based metal powder is a main component exhibiting electric conductivity. Examples of the zinc-based metal powder include a metal zinc powder, brass powder and tin-zinc powder. Examples of the configuration of each powder include spherical, elliptical, and flaky configurations. It is preferable that the surface of each powder is provided with a large number of irregularities to obtain an increased specific surface area. With the increased specific surface area, the molten low melting metal can be captured firmly, and consequently, the phase separation of the molten low melting metal can be prevented effectively. Zinc is wetted well with the low melting metal, particularly tin, and also is not alloyed with tin at a high rate. Consequently, tin wetted with and adhering to the surface of zinc is only slightly alloyed with zinc to be absorbed therewith so that such tin exists on the surface of zinc for a long time as molten tin or a tin alloy, which is favorable for joining the zinc-based metal powder.
The preferred particle diameter of the zinc-based metal powder is about 1 to 100 xcexcm, and more preferably about 15 to 80 xcexcm . As the particle diameter decreases, oxidation film increases to worsen the current-carrying properties, whereas as the particle diameter increases, the dispersing properties thereof lowers, and consequently, both the moldability and strength decrease.
The preferred zinc-based metal powder content is about 10 to 60 volume % per 100 volume % of the overall electroconductive resin composition. If good electric conductivity is required, the preferred content is 30 volume % or more, and more preferably 40 volume % or more. If molded products are required to have increased strength, the preferred zinc-based metal powder content is 40 volume % or less.
To strengthen joining of the zinc-based metal powder and resin, it is also preferable to treat the surface of the zinc-based metal powder with a silane-based or titane-based coupling agent.
Tin alloys normally known as solder can be used as the low melting metal. More specifically, tin, tin-zinc, tin-copper, tin-indium, tin-silver alloys or the like can be used. Normally, the low melting metal is added to the resin as a fine powder. The preferred particle diameter of the low melting metal is about 6 to 50 xcexcm.
The low melting metal content is about 3 to 30 volume %, and more preferably 6 to 15 volume %, per 100 volume % of the overall electroconductive resin composition.
Thermoplastic resins can be used as the synthetic resin material. More specifically, crystalline resin such as 12-nylon, 6-nylon, 66-nylon, polyacetals, polyethylene terephthalate(PET), polybutylene terephthalate(PBT), polyphenylene sulfide(PPS), polystyrene, syndiotactic polystyrene(SPS), polypropylene(PP), polyethylene(PE) and ethylene copolymerized resin (EVA, EAA, ionomer), non-crystalline resins such as ABS, polyurethane, polycarbonate(PC), and renatured polyphenylene oxide resin, liquid crystal polymer, and thermoplastic elastomer can be used. In addition, polymer alloys formed by blending at least one of these thermoplastic resins together may be used. In special cases, a rubber material such as silicon rubber, fluorine rubber, acrylic rubber and the like, epoxy resin, pherolic plastic and thermosetting resin can be used.
The preferred synthetic resin material content is about 30 to 80 volume %, and more preferably 40 to 65 volume %, per 100 volume % of the overall electroconductive resin composition.
The preferred relative composition ratio of the zinc-based metal powder to low melting metal is about 1 to 20 in volume, and more preferably about 4 to 10 in volume. If the relative composition ratio decreases, low melting metals readily cohere to each other to worsen the moldability. If the relative composition ratio increases, the electric conductivity and electromagnetic shielding tend to decrease.
In addition, a metal fiber, metal powder other than the zinc-based metal powder, filler such as calcium carbonate or talc, and other additives normally used in preparing resin compositions can be added.
The preferred metal fiber is an aluminum-based metal fiber composed of aluminum, an aluminum alloy or the like. The preferred diameter of the metal fiber is about 50 to 100 xcexcm, and the preferred length thereof is about 2 to 5 mm to effect good moldability and electric conductivity. Where the aluminum-based metal fiber is used as the metal fiber, the preferred content of the zinc powder and aluminum-based metal fiber is about 20 to 50 volume % per 100 volume % of the overall electroconductive resin composition.
The preferred volume ratio of the total of the zinc powder and aluminum-based metal fiber to the low melting metal, that is, (volume of zinc powder+volume of aluminum-based metal fiber)/volume of the low melting metal, ranges from 2 to 17. When this ratio increases further, the electric conductivity becomes worse, whereas when this ratio decreases further, the separation of the low melting metal occurs.
The electroconductive resin composition can be produced by melting and kneading the above-described components with an extruder or the like into pellets. By injection molding of the resultant electroconductive resin composition as a raw material, a resin molded product exhibiting a desired electric conductivity can be obtained. In particular, the electroconductive resin composition of the present invention is favorable as a material for use in electric circuits. Excellent resin-molded electric parts can be obtained by molding a substrate with a non-electroconductive resin composition, and molding an electric circuit with the electroconductive resin composition of the present invention on the substrate integrally therewith by two-step molding.
In the cases only the electric circuit is formed using the electroconductive resin by two-step molding, it is also preferable to increase the content of metal components such as the zinc-based metal powder and low melting metal, and consequently, enhance the electric conductivity of the resultant electric circuit further.
If resultant molded parts are required to effect electromagnetic shielding or exhibit thermal conductivity, the synthetic resin material content can be increased while decreasing the metal components content to obtain necessary strength. To mold gaskets having electromagnetic shielding properties, rubber or elastomer can be used as the synthetic resin material.
With the electroconductive resin composition of the present invention, the zinc-based metal powder acts as a main component of an electroconductive material thereof. The zinc-based metal is relatively soft as a metal material, and the melting point thereof is low. In addition, the zinc-based metal powder is well wetted with an tin alloy as a low melting metal. This results in the molten low melting metal adhering to the surface of the zinc-based metal powder to be captured therewith so that the phase separation of the molten low melting metal from both the resin and zinc-based metal powder is reduced. The zinc-based metal powder does not have a fiber-like configuration, but has a relatively round configuration, as compared to the fiber so as not to obstruct the flow of the molten resin greatly. Thus, the electroconductive resin composition of the present invention has good moldability.
It is expected that if the metal fiber is replaced with the metal powder, the electric conductivity of resultant molded bodies would be degraded thereby. But, actually, the electric conductivity thereof is not degraded greatly, which does not cause any practically great problem. To overcome the degradation of the moldability, which has been encountered with the case a large amount of metal fiber is contained, the amount of the resin component need be increased. In contrast, with the use of the zinc-based metal powder, the moldability is enhanced so that the composition ratio of the zinc-based metal powder can be increased while reducing the composition ratio of the resin component. This results in the degradation of the electric conductivity due to the use of the zinc-based metal powder being able to be covered.
The electroconductive resin composition in accordance with the present invention can be applied to electric circuits by two-step molding, resin molded products having electromagnetic shielding function, gaskets for use in electromagnetic shielding, or the like.
Other objects, features, and characteristics of the present invention will become apparent upon consideration of the following description and the appended claims with reference to the accompanying drawings, all of which form a part of this specification.